A screw pump comprising two externally threaded rotors mounted in a pump body and adapted for counter-rotation in the body with intermeshing of the rotor threads is well known. Close tolerances between the rotor threads at the points of intermeshing and with the internal surfaces of the pump body cause volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface of the pump body and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive because they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the pump inlet down to atmospheric pressure at the pump outlet. As a result, a screw pump may be employed as a backing pump for a secondary pump, such as a turbomolecular pump, for evacuating a process tool.
If a screw pump has a sufficiently high capacity, two turbomolecular pumps could be simultaneously backed by a single screw pump by connecting the exhausts of the turbomolecular pumps to the inlet of the screw pump via a common backing line. However, in such an arrangement, any variation in the flow rate of pumped gas exhaust from one of the turbomolecular pumps could change the fluid pressure within the common backing line, which in turn would affect the performance of the other turbomolecular pump exhausting into the common backing line. In view of this, typically each secondary pump is backed by a respective pump.
It is an aim of at least the preferred embodiment of the present invention to provide an improved screw pump which can simultaneously back two pumps whilst substantially avoiding the aforementioned problem.